1. Field of the Invention
The present invention relates to a method and apparatus for reading out address information from an optical disc medium, on which the address information was recorded or represented by varying the shape of a track groove.
2. Description of the Related Art
A groove recording technique is one of known methods of recording or representing address information on a recording track of a recordable or rewritable optical disc. In the groove recording technique, the track groove of an optical disc to record an optical signal thereon is wobbled and the frequency, phase or amplitude of the wobbled groove is changed, thereby recording or representing address information on the groove.
FIG. 1 illustrates various shapes of wobbled grooves 100 through 103. The wobble of the groove 100 consists essentially of a sine wave alone. The groove 101 also has a sine wave wobble but includes a phase inverted portion as well. ISOM 2001 Technical Digest, pp. 6–7, discloses the technique of representing address information by the grooves 100 and 101. More specifically, according to the technique, data “0” and data “1” are represented by the non-inverted phase and the inverted phase of the sine wave, respectively.
The groove 102 shown in FIG. 1 has a wobbled pattern, of which the displacements toward the center of the disc are relatively steep with respect to the tracking direction and of which the displacements away from the center of the disc are relatively gentle with respect to the tracking direction. Contrarily, the groove 103 shown in FIG. 1 has a wobbled pattern, of which the displacements away from the center of the disc are relatively steep with respect to the tracking direction and of which the displacements toward the center of the disc are relatively gentle with respect to the tracking direction. In FIG. 1, the downside is closer to the center (or the lead-in area) of the disc, while the upside is closer to the edge (or the lead-out area) of the disc. PCT International Application No. WO 01/52250 discloses the technique of representing address information by the grooves 102 and 103. More specifically, according to the technique, data “0” and data “1” are allocated to the grooves 102 and 103, respectively.
FIG. 2A is a block diagram showing a circuit for reading out an address read signal and a user data read signal from an optical disc by recognizing the various shapes of the wobbled groove. FIG. 2B schematically shows how a laser beam 201 scans the groove 200 on the optical disc. Specifically, the laser beam 201 is focused onto, and reflected from, the groove 200 of the optical disc. Then, the reflected beam forms a beam spot 202 over the detectors 203 and 204 of the circuit. As shown in FIG. 2A, the detectors 203 and 204 are spaced apart from each other in a radial direction of the disc. Each of the detectors 203 and 204 supplies an output signal, representing the intensity of the received laser beam, to a differential amplifier 205. On receiving the output signals of the detectors 203 and 204, the differential amplifier 205 calculates a difference between these two signals, thereby outputting an address read signal 206. In this manner, a wobble signal 206 representing the shape of the groove 200 can be obtained as shown in FIG. 2B. The outputs of the detectors 203 and 204 are added together by an adder 207 to produce a user data read signal 208.
When data is read out from an optical disc, the quality of the read signal may deteriorate for various reasons including: interference between adjacent tracks; variation in the tilt angle defined by a laser beam with respect to the disc surface of the optical disc; and dust or scratches on the data recording side. In that case, the quality of the address read signal may also deteriorate and the address information may not be detected accurately. Particularly when the optical disc drive used can write, such inaccurate detection of address information may lead to erroneous erasure of user data that has already been recorded on the disc.